The present invention relates generally to capacitors and, more particularly to metallized film capacitors especially suited for use in pulse applications.
The use of metallized electrodes evaporated onto capacitor dielectric film is an advantageous way to achieve small film capacitor size for a given capacitance and voltage. Although many configurations are possible using metallized film, a configuration which uses two dielectric films and two metallized electrode layers is generally regarded as an optimum design for a given size, capacitance and voltage withstand ratings. For example, two oblong dielectric films are each coated with a metallized electrode layer. The dielectric films may include a longitudinal direction and a transverse direction such that the dimension of the dielectric films in the longitudinal direction is longer than that in the transverse direction. The two dielectric films are disposed in an overlapping fashion such that the metallized electrode layers of the two dielectric films are not in contact. Then, the overlapping films are wound together to form a roll including alternating, offset layers of the metallized film. The roll then becomes the main body of the capacitor.
The connection to the metallized electrodes is generally made by spraying molten metal droplets at the ends of capacitors wound with alternating offset layers of the metallized film. Unfortunately, this connection is not usually homogenous nor continuous. The first droplets to hit the edge of the capacitor film splatter onto the extension volume where they solidify and come into contact with the metallized film. These splattered metal droplets serve as the electrical and mechanical connection to the metallized film. The ends of the capacitor are built up with sprayed metal until there is sufficient thickness for attaching lead wires thereon. Since the capacitor current is carried entirely through the splattered droplets, the current density in the metallization adjacent to those connection points is very high. When current density becomes too intense around a particular connection point, the metallization layer surrounding the connection point tends to become at least partially vaporized, thereby destroying the connection point.
The AC power industry uses very large capacitors for power factor correction. These large capacitors store a large amount of energy, and, if a dielectric fault occurs, a large amount of internal capacitor damage is possible if the current is not limited. The use of capacitors (minimally sized using the above described, traditional construction) in pulse applications has been limited by the capability of the connection points to carry current between the metallization and the end spray metal without vaporizing the metallization layer around these contact points due to the high current density. The problem is not so much that some of the connections disappear, but that, as each connection point is removed, the current in the adjacent connection points increases. This increase in current at adjacent connection points tends to initiate a runaway failure mode, in which each subsequent discharge removes a substantial number of the connection points. As these connection points are lost along a substantial length of the film, the capacitance quickly becomes extremely lossy because current is forced to flow along the length of the metallization layer to the nearest contact location. This current flow along the length of the metallization will degrade the quality of the capacitance. The damaged capacitor cannot be discharged as quickly, and the available discharge current consequently drops rapidly, thus leading to catastrophic failure of the entire capacitor. As a result, applications that depend on very fast discharge at high current can fail quickly and without warning within a very few discharges.
Turning now to the drawings, wherein like components are indicated by like reference numbers throughout the various figures where possible, attention is immediately directed to FIGS. 1A–1D, which illustrate the mechanism of runaway failure due to vaporization of the metallization layer around the connection points. It is noted that the various components shown in the figures are not shown to scale for purposes of illustrative clarity. FIG. 1A presents a magnified view of a capacitor 1A including an interface between a metallized film 10 and an end spray metal 12. A plurality of connection points are indicated by marks 15. A plurality of arrows 20A indicates current flow direction upon application of a first discharge pulse. Attention is particularly directed to one of the plurality of connection points 15 indicated by a letter P. This small connection point P is a weak link between metallized film 10 and end spray metal 12, as will be described in detail immediately hereinafter.
As shown in FIG. 1B, the application of the first discharge pulse results in vaporization of the metallization immediately surrounding connection point P, thereby resulting in a connection failure 30 around connection point P. Current flow direction (indicated by arrows 20B) thereby shifts to skirt around connection failure 30 upon application of a second pulse discharge. In particular, the density of current flow (indicated by the concentration of arrows 20B) around connection failure 30 increases, consequently causing further damage to the metallization surrounding connection failure 30.
FIG. 1C illustrates such increasing damage to the metallization layer surrounding connection point P. Particularly, connection failure 30 of FIG. 1B has expanded in area into an increased, connection failure 30′. Current flow direction (indicated by arrows 20C) in the presence of a third pulse discharge now skirts around increased connection failure 30′ such that the density of current flow (indicated by the concentration of arrows 20C) is further increased around connection failure 30′. The increased density of current flow around connection failure 30′ leads to further damage to the metallization such that the connection failure still increases in size (as indicated by reference numeral 30″ in FIG. 1D) with successive pulse discharge. It is noted that the amount of damage to the metallization layer due to this effect is dependent upon the size of pulse current. With large pulse currents, each occurrence of pulse discharge may lead to vaporization of a significant length of metallization, thereby quickly leading to substantially complete disconnection of metallization layer 10 from end spray metal 12 and, consequently, to capacitor failure.
One way to improve the performance of traditional design capacitor films in pulse application is to make the capacitors short, with the diameter being equal to or larger than the length of the rolled capacitor film. In this way, the short capacitor would include a long edge onto which the end spray metal may come into contact such that, for a given pulse current, the current carried per unit length of film is smaller in the short capacitor than in the traditional capacitor. Consequently, the current carried through each connection point between the end spray metal to the metallization is reduced. For a given capacitance and film thickness, the current carried per unit film length increases as the form factor of the capacitor is varied toward longer length and smaller diameter. However, the limited availability of space in many applications generally make longer capacitors more desirable as they make better use of available volume.
In general, the capacitor manufacturing process must be very well controlled because, if at some location the contact of the end spray metal to the metallization layer is inadequate to carry its share of the discharge current, that location becomes the initiation point for catastrophic capacitor failure as described above. This problem may be alleviated by making the edge of the metallization, which serves as the end onto which the end spray metal makes contact, thicker than the metallization of the remaining active part of the capacitor. This so-called “heavy-edge” metallization is a major improvement over the traditional capacitor design because it significantly increases the allowable current density before failures occur. However, the heavy-edge in itself does not change the mechanism of capacitor runaway failure. As a result, the pulse current capability of the capacitor with a heavy-edge metallization remains dependent on the manufacturing process. In other words, heavy-edge metallization has been used to allow end spray metal to better connect to the metallized film edge when thinner metallization is used in the active area to optimize capacitor voltage withstand.
In prior art capacitors, the heavy-edge is created by depositing more metal (such as aluminum) at the film edge in comparison to the active region of the capacitor, or by separately depositing a second metal (such as zinc) only at the film edge so as to result in the heavy-edge that will safely carry higher current therethrough in comparison to a capacitor without the heavy-edge structure. Metallization thickness is commonly quantified by the surface resistivity in units of ohms/square. It is known that there is a large difference in resistivity between different materials, such as between aluminum and zinc. A zinc layer of a certain resistivity value would be several times thicker than a layer of aluminum of equivalent resistivity. Therefore, one may achieve a thicker heavy-edge by separately depositing, for example, a zinc layer along the edge of a very thin, aluminum layer active region (as taught by Unami et al. in U.S. Pat. No. 5,696,663 (hereinafter Unami)). Alternatively, (different from Unami's teachings) the active region and the heavy-edge may be formed of a single alloy, but this technique is substantially limited in the current art because it is believed that one cannot simultaneously achieve a thin enough active region for self-healing and a thick enough heavy-edge for improved contact with the end spray metal by using a single alloy. As another alternative, it is recognized that making two passes with aluminum may also be used to build up the heavy-edge but allowing a large heavy edge to body ratio after the second pass. However, this approach does not mitigate the problems inherent in pulse applications because the resulting aluminum heavy-edge is too thin and therefore does not lead to sufficiently decreased current density. The present state of the art zinc alloy heavy-edge has a ratio limit between the metallization thickness in the active area and the metallization thickness along the heavy-edge. It is submitted that this limit in thickness ratio prevents simultaneous optimization of pulse current capability and voltage withstand. The exact details of the heavy-edge fabrication is known in the art and is considered to be outside the scope of this application.
Another variation in film capacitor design is segmenting of the metallization layer in specific ways. Segmentation was initially conceived as a method to allow disconnection of a defective segment, such as a segment including a dielectric fault, from the end spray metal. The segmentation prevents current within the film capacitor from traveling along the length of the film toward the fault site. The segmentation also forces the fault current to flow through the connection from the end spray metal to the defective segment. However, the ability of conventional segmentation schemes to control fault current via the end spray metal to metallization is poor due to the close dependence of the connection quality to the manufacturing process variability. More elaborate segmentation patterns which use narrow “bottleneck” configurations to limit current have also been devised. For instance, if too high a current flowed through one of these bottleneck segments, the metallization in the bottleneck would vaporize and essentially act as a fuse, thus preventing internal current from reaching the fault location. The use of such segmentation schemes are advantageous because the thickness and patterning of the metallization layer is much more easily controlled than the connection of the end spray metal to the metallization layer.
The effect of segmentation metallization on reducing runaway capacitor failure is illustrated in FIGS. 2A and 2B. Like capacitor 1A–1D of FIGS. 1A–1D, a capacitor 100A of FIG. 2A includes end spray metal 12. However, capacitor 100A also includes a segmented metallization film 110A, with each segment being spaced apart from every other segment by a separation 111. A plurality of marks 115 indicate a plurality of connection points. Direction of current flow upon application of a first pulse discharge is indicated by a plurality of arrows 120A. In particular, a high current density (indicated by the concentration of current flow 120A) around a small connection point P leads to vaporization of the metallization around connection point P, as shown in FIG. 2B. However, due to the plurality of separation 111, the connection failure is isolated to that segment containing connection point P, and application of additional pulse discharge does not necessarily result in further damage to segment metallization film 110B. Therefore, connection failure around connection point P does not propagate beyond that segment containing connection point P even with application of additional pulse current.
An example of metallization segmentation is presented by aforementioned Unami, which describes the patterning of the metallization layer in the longitudinal direction such that the electrode metallization is in the form of long strips in the longitudinal direction on the dielectric films. By offsetting the pattern on the two films forming the capacitor main body, the capacitor of Unami effectively acts as a plurality of sub-capacitors placed in series (see, for example, FIGS. 4A–4D of Unami). Unami also suggests segmenting the metallization layer in the transverse direction as well as the longitudinal direction such that the capacitor is composed of a plurality of sub-capacitors placed in parallel as well as in series (see, for example, FIG. 5 of Unami). It is submitted that the segmentation is used solely to help isolate dielectric faults in capacitors designed to best use the film dielectric strength (i.e., to achieve the highest possible voltage rating for a given film thickness). In both cases of segmentation patterns as discussed in Unami, the metallization also includes a heavy-edge formed by separately depositing a zinc edge film on top of an aluminum film forming the active region. That is, Unami discloses the use of an aluminum layer as the active region of the metallization and a separately deposited, zinc film forming the heavy-edge in order to make the active region thin enough for self-healing (i.e., vaporization of the metallization film around an insulation defect) to take place while the heavy-edge is made thick enough for improved contact with the end spray metal. In particular, Unami requires that the aluminum, active region exhibit a resistivity of 8 ohms per square or greater, while the heavy-edge be configured to exhibit a resistivity of 1.5 to 7 ohms per square (see, for example, column 7 line 59 to column 8 line 23 of Unami), with the lower resistivity of 1.5 ohms or 2 ohms per square not being materially important to the Unami design. Unami explains that this two-step metallization process, using two different metals, is necessary because it is difficult to attain the aforedescribed combination of thickness/thinness properties using the same material for the heavy-edge and the active region (see, for example, column 2 line 66 to column 3 line 6 of Unami). Unami is basically concerned with increasing the voltage withstand of the capacitor, rather than to optimize pulse current for pulse operation. In fact, the resistivity recommendations of Unami, as justified in FIG. 16 of Unami, show that if the heavy-edge is made too thick (i.e., thicker than a thickness yielding 2 ohms/square), the capacitor voltage withstand capability may be compromised.
The present invention provides a capacitor and associated method which serves to reduce or eliminate the foregoing problems in a highly advantageous and heretofore unseen way and which provides still further advantages.